1. Field of the Invention
The invention relates to screws used in medical applications to affix medical implants, allografts, autografts or heterografts to bone structure of a patient. More specifically, the invention provides polymeric medical fixation screws with multiple cannulae for receiving means for applying torquing to the screws, such that the screws are subjected to reduced stresses while being torqued into place, thereby reducing the risk of shearing the screws.
2. Description of the Related Art
Medical screws of various designs and materials are used to affix medical implants, autografts, allografts or heterografts to bone structure of a patient. Depending upon the particular application, the medical screw may be headless (an interference screw) or be supplied with a suitable head (such as screws used to fix bone plates in place, anchor soft tissue to bone, and like applications). Medical screws are typically fabricated from biocompatible metal alloys or biocompatible organic polymers. Polymeric materials provide significant advantages over metal alloys in certain specific circumstances. In particular, when the polymer is bioabsorbable, then the requirement of a second surgical procedure to remove the screw, and any other temporarily needed implant device, is eliminated. Further, bioabsorbable implants and screws can be designed to facilitate improved bone healing by gradually transferring applied loads from a fixation device to the natural bone, as the device's material properties degrade at a predetermined rate. Therefore, the use of bioabsorbable medical screws is desirable in a wide range of applications.
Bioabsorbable screws being made of organic polymers, however, often have insufficient strength to withstand shear loads that must be applied during the insertion of the screw into a hole in bone. Efforts have been made to alleviate the shear stress problem by supplying the bone screws with heads having a recess for receiving an insertion tool with a non-circular cross-section, such as an hexagonal recess and matching tool. This screw may also be supplied with a bore for receiving a guide pin extending from the recess through the entire length of the screw to guide the screw into place. However, only the recess in the head of the screw is designed for torque transfer. Thus, stresses are concentrated in the vicinity of the head of the screw so that it is susceptible to shear failure in this region when it is being torqued into place.
Another approach, shown in U.S. Pat. No. 5,169,400, is a screw designed with a single, central non-circular cross-section cannula extending from the screw head, along the shank, to the tip. The non-circular cross-section of the cannula is essential to allow torque transfer. The cannula is disclosed as hexagonal, square, or star-shaped in cross-section. The cannula receives a torquing tool and the design is intended to distribute torque force over the entire length of the screw shaft when the screw is being torqued into place. Nevertheless, this design suffers from several disadvantages. These include peak tensile, compressive, and shear stresses located at adjacent sides of each corner within the cannula posing a significant risk of screw failure, especially since the corners coincide with the thinnest section of the screw body.
It is desirable to develop a polymeric medical screw that can be used to affix medical implants or grafts to the bone structure of a human patient. The screw should be able to withstand torquing force encountered in driving the screw into place in a hole in bone, with minimal risk of shear failure. Further, the screw should be convenient to use during surgical procedures.